1. Field of the Invention
The present invention relates to pharmaceutical compositions comprising sustained-release opioid receptor antagonists and methods of administration and use thereof
2. Description of the Related Art
Obesity is a major health concern in Western societies. It is estimated that about 97 million adults in the United States are overweight or obese. Epidemiological studies have shown that increasing degrees of overweight and obesity are important predictors of decreased life expectancy. Obesity causes or exacerbates many health problems, both independently and in association with other diseases. The medical problems associated with obesity, which can be serious and life-threatening, include hypertension; type 2 diabetes mellitus; elevated plasma insulin concentrations; insulin resistance; dyslipidemias; hyperlipidemia; endometrial, breast, prostate and colon cancer; osteoarthritis; respiratory complications, such as obstructive sleep apnea; cholelithiasis; gallstones; arterioscelerosis; heart disease; abnormal heart rhythms; and heart arrythmias (Kopelman, P. G., Nature 404, 635-643 (2000)). Obesity is further associated with premature death and with a significant increase in mortality and morbidity from stroke, myocardial infarction, congestive heart failure, coronary heart disease, and sudden death.
Obesity is often treated by encouraging patients to lose weight by reducing their food intake or by increasing their exercise level and therefore increasing their energy output. A sustained weight loss of 5% to 10% of body weight has been shown to improve the co-morbidities associated with obesity, such as diabetes and hypertension, and can lead to improvement of obesity-related conditions such as osteoarthritis, sleep apnea and pulmonary and cardiac dysfunction.
Naltrexone, having the chemical name (17-(cyclopropylmethyl)-4,5α-epoxy-3,14-dihydroxymorphinan-6-one) and the chemical structure shown below, is an opioid receptor antagonist used primarily in the management of alcohol dependence and opioid dependence.

Naltrexone for oral administration has been commercially available for a number of years from various sources as the hydrochloride salt, naltrexone hydrochloride, e.g., under the trade names REVIA™ (50 mg) and DEPADE™ (25 mg, 50 mg and 100 mg). The currently approved forms of oral naltrexone are immediate release formulations that are efficacious even when dosed as infrequently as once every 72 hours. For example, the label of the DEPADE® brand of naltrexone hydrochloride indicates that naltrexone is a potent opioid antagonist with a prolonged pharmacological effect (24 to 72 hours) and recommends a dose of 50 mg once daily. The DEPADE® label discloses that clinical studies indicate that 50 mg of naltrexone hydrochloride will block the pharmacologic effects of 25 mg of intravenously administered heroin for periods as long as 24 hours. The DEPADE® label goes on to indicate that other data suggest that doubling the dose of naltrexone hydrochloride provides blockade for 48 hours, and tripling the dose of naltrexone hydrochloride provides blockade for about 72 hours. Thus, despite reaching a peak serum concentration quickly (Tmax of approximately 1 hour) after oral administration, the immediate release form of naltrexone has relatively long lasting effects.
The long-lasting effects of the immediate release form of oral naltrexone may be used to encourage patient compliance by utilizing a dosing frequency that is less than once per day, e.g. every other day or every three days. For example, the DEPADE® label indicates that a flexible approach to a dosing regimen may be employed to enhance compliance. Thus, the DEPADE® label discloses, patients may receive 50 mg of naltrexone hydrochloride every weekday with a 100 mg dose on Saturday or patients may receive 100 mg every other day, or 150 mg every third day. The DEPADE® label refers to clinical studies reported in the literature that have employed the following dosing regimen: 100 mg on Monday, 100 mg on Wednesday, and 150 mg on Friday. Thus, use of the immediate release oral form allows a patient to take a relatively large dose of naltrexone at a time when the temptation to abuse alcohol or opioids may be less (e.g., during the week), with the effects lasting to a time when temptation may be greater (e.g., over the weekend).
The DEPADE® label indicates that naltrexone has not been shown to cause significant increases in complaints in placebo-controlled trials in patients known to be free of opioids for more than 7 to 10 days. Although a subset of the patient population reports nausea upon initial administration of 25 mg or 50 mg dosages of the immediate release oral form of naltrexone, the nausea often subsides as those patients develop a tolerance. The DEPADE® label indicates that, in an open label safety study with approximately 570 individuals with alcoholism receiving naltrexone, the following new-onset adverse reactions occurred in 2% or more of the patients: nausea (10%), headache (7%), dizziness (4%), nervousness (4%), fatigue (4%), insomnia (3%), vomiting (3%), anxiety (2%) and somnolence (2%). Moderate to severe nausea was reported by 18 patients (15%) in a 10-week open label study of naltrexone among alcohol dependent participants, involving a initial 25 mg dose followed by a dose of 50 mg daily for 10 weeks (O'Malley, S. S. J. Clin. Psychopharmacol. 2000 February; 20(1):69-76). Eight of the eighteen patients that experienced moderate to severe nausea discontinued treatment, nausea resolved within one week for five subjects and within two weeks for four, and it continued off and on throughout the ten-week period for one subject. The authors linked the moderate to severe nausea to poorer medication compliance and heavier drinking by the patients during treatment.
It appears that while some patients experience adverse effects upon administration of 25 mg or 50 mg dosages of immediate release oral naltrexone, the adverse effects often subside within a relatively short period of time and many patients are able to tolerate significantly higher oral dosages, e.g., 100 mg or 150 mg. A once-monthly injectable form of naltrexone is commercially available under the tradename VIVITROL® for the treatment of alcoholism. The prescribing information for VIVITROL® indicates that patients should be advised that they may experience nausea following the initial injection of the VIVITROL® naltrexone; that these episodes of nausea tend to be mild and subside within a few days post-injection, and that patients are less likely to experience nausea in subsequent injections.
U.S. Patent Publication No. 2004/0254208 A1 discloses the use of naltrexone in combination with other compounds for affecting weight loss, but does not disclose any particular adverse effects associated with the combinations.